Hannestad : Can Dust Segregation Mimic a Cosmological Constant ?

Recent measurements of type Ia supernovae indicate that distant supernovae are substantially fainter than expected from the standard flat cold dark matter model. One possible explanation is that the energy density in our universe is in fact dominated by a cosmological constant. Another possible solution is that there are large amounts of grey dust in the intergalactic medium. Dust grains can be grey either because they are non-spherical or very large. We have numerically investigated whether grey dust can be emitted from high redshift galaxies without also emitting standard, reddening dust, which would have been visible in the spectra of high redshift objects. Our finding is that grain velocities are almost independent of ellipticity so that if greyness are due to the grains being elongated, it will not be possible to separate grey dust from ordinary dust. We also find that velocities are fairly independent of grain size, but we cannot rule out possible sputtering of small grains, so that large, grey dust grains could be preferentially emitted. Therefore, our conclusion is that grey dust is an unlikely explanation of the data, but we cannot rule it out if the grey dust consists of large, spherical grains.